MIMO Networks
This invention considers multiple-input multiple output (MIMO) wireless networks comprised of one or more base stations (BSs). Each BS serves a set of mobile stations (MSs) in a cell associated with the BS. In most practical wireless network standards, including IEEE 802.16m, closed-loop (CL) precoding is defined for down link (DL) transmission from the BS to each MS by exploiting codebooks shared by all BSs and MSs. However, due to the fact that these codebooks are designed for uncorrelated MIMO channels, they are suboptimal for correlated MIMO channels.
To cope with this problem, a method using adaptive codebooks is commonly adopted. This method enables the BS to optimize the DL precoding by exploiting feedback of quantized channel state information (CSI), e.g. either long-term channel covariance matrix or the instantaneous MIMO channel matrix, from the MS.
Generally speaking, this method is applicable to both multi-carrier and single-carrier systems. For presentational clarity, This description concentrates on multi-carrier systems using orthogonal frequency division modulation (OFDM) in this invention. Thus, the long-term channel covariance matrix over the jth subcarrier is commonly defined asR=E{HijHijH},where E{·} stands for an expectation of the enclosed quantity, and Hij is the correlated channel matrix for the ith OFDM symbol and jth subcarrier.
However, this quantized feedback has a large feedback overhead. Exploiting the fact that the covariance matrix R is Hermitian (H) with complex entries equal to a conjugate transpose of entries in the matrix, the existing solution is to concentrate on only diagonal and upper-triangular elements, while quantizing the elements with different levels of precision. Unfortunately, the feedback overhead reduction provided by this direct quantization method remains rather large. For instance, the IEEE 802.16m standard requires 28 bits to feed back a 4×4 matrix R by quantizing each real-valued diagonal element and complex-valued upper-triangular element with one and four bits, respectively. Thus, more efficient quantized feedback methods with significantly reduced feedback overhead are desired.
In addition to large feedback overhead, the existing quantized feedback method suffers from asymmetric interference. For the MSs near the BS, the above definition of R is sufficient for the BS to optimize the DL precoding. However, the MSs near the edge of the cell are subject to asymmetric interference from the BS and the MSs in adjacent cells. As a result, the DL transmission based on the long-term channel covariance matrix defined above is suboptimal due to the fact that it ignores inter-cell interference (ICI).